1. Field of the Invention
The present invention relates to an image forming apparatus for forming a plurality of dots in the primary and secondary scanning directions.
2. Description of the Related Art
Conventional electrophotographic image forming apparatus are classified into a scanning type and a non-scanning type. In a scanning type apparatus, the surface of the photosensitive drum is scanned by a laser beam in the primary scanning direction to produce electrostatic latent images. To move the laser beam in the primary scanning direction, the scanning type apparatus may include a rotatable mirror for reflecting the beam. In a non-scanning type apparatus, on the other hand, use is made of an optical print head provided with a plurality of light-emitting diodes arranged in an array extending in the primary scanning direction. In operation, these light-emitting diodes are selectively activated based on one-line image data, so that the latent image corresponding to the one-line data is produced on the photosensitive drum.
A non-scanning type apparatus may be provided with more than one optical print head for enabling the switching of image-forming resolution. For example, three optical heads may be used for allowing the user to select one from three resolution modes (e.g. fine, intermediate and coarse). In this manner, however, the apparatus tends to be expensive and bulky due to the plural print heads.
JP-A-5(1993)-238062 and JP-A-6(1994)-336054 disclose non-scanning type image forming apparatus, in which use is made of only one optical print head, but still the switching of resolution is possible. For enabling the resolution switching, the apparatus of JP-A-5(1993)-238062 is provided with a refractivity-variable member disposed between the print head and the photosensitive member, while in the apparatus of JP-A-6(1994)-336054, the optical print head is designed to be moved toward or away from the photosensitive member.
The use of the above refractivity-variable member, however, is disadvantageous to achieving the downsizing of the image forming apparatus since extra space is needed for providing the refractivity-variable member. The movable print head of JP-A-6(1994)-336054 is also disadvantageous since a precisely-build driving mechanism is required for moving the print head relative to the photosensitive member.
The present invention has been proposed under the circumstances described above, and its object is to provide an image forming apparatus capable of switching from one resolution mode to another without causing an increase in cost and size.
According to a first aspect of the present invention, there is provided an image forming apparatus for producing an array of dots in a primary scanning direction based on image data prepared for one line. The apparatus includes: enlarging means for enlarging the image data n times in the primary scanning direction to form images with a resolution equal to (1/n) times a maximum resolution, where the n is a positive integer; and dot forming means for forming dots for n lines in a secondary scanning direction based on the enlarged image data.
Based on the same image data for one line, the dots for n lines are arranged in an identical pattern in the secondary scanning direction.
According to a second aspect of the present invention, there is provided an electrophotographic image forming apparatus including: an optical print head provided with an array of light-emitting elements extending in a primary scanning direction; a photosensitive member on which electrostatic latent images are produced; enlarging means for enlarging one-line image data n times in the primary scanning direction to form images with a resolution equal to (1/n) times a maximum resolution, where the n is a positive integer; and radiation controlling means for actuating the light-emitting elements n times with a predetermined actuation cycle based on the enlarged one-line image data.
The photosensitive member may be a photosensitive drum or photosensitive belt. The enlarged one-line image data may be supplied from the enlarging means to the print head with a data supply cycle equal to n times a minimum data supply cycle.
In a preferred embodiment of the present invention, the actuation cycle may be invariant as the positive integer n varies.
According to a third aspect of the present invention, there is provided an electrophotographic image forming apparatus including: an optical print head provided with an array of light-emitting elements extending in a primary scanning direction; a photosensitive member on which electrostatic latent images are produced; enlarging means for enlarging one-line image data n times in the primary scanning direction to form images with a resolution equal to (1/n) times a maximum resolution, where the n is a positive integer; and radiation controlling means for actuating the light-emitting elements with an actuation cycle equal to n times a minimum cycle based on the enlarged one-line image data.
Preferably, the minimum cycle may be used for performing maximum resolution printing. The light-emitting elements may be held in a light-emitting state for a time span corresponding to the positive integer n. This time span may be equal to n times a minimum time span.
According to the present invention, there is no need to prepare any optical parts or mechanical structure for performing the switching of resolution.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.